The present invention relates to a method and a device serving to detect an internal arc in a metal-clad electrical link, in particular for links that are long, e.g, 100 meters (m) in length.
When a short-circuit current occurs in a metal-clad electrical link, an increase in the pressure inside the cladding is generated at the fault, and that pressure increase results in two pressure surges being generated, each of which propagates in a respective direction away from the place where the short-circuit current has occurred. By detecting the pressure surge, it is possible to observe the appearance of a fault in the line. It is particularly advantageous to locate the place at which the fault occurs so as to detect faults that are repetitive.
Currently, the pressure surge generated by the short-circuit current is detected by measuring the increase in the mean pressure by means of a pressure sensor. For this purpose, it possible to use density transmitters that serve to monitor the density of the gas inside the cladding segment in question, each such density transmitter being constituted by a pressure sensor whose measurement is compensated as a function of temperature so as to obtain the density of the gas.
The pressure surge that is generated is relatively small, e.g. about 10%, and the pressure surge that propagates inside the cladding from the fault point is attenuated as it propagates and its value decreases going away from the fault point. As a result, the sensitivity of those detectors is low, and it is not always possible to detect faults that occur in segments of long length.
The problem addressed by the invention is to improve detection sensitivity and to determine the position of the short-circuit arc relative to the detector.
In the present invention, the rising edge of the internal pressure surge generated by the arc and propagating inside the cladding is detected at a fixed location along the cladding.
It is easier to detect the passage of the rising edge of a pressure surge because its amplitude is greater than the mean pressure variation, and this significantly increases the sensitivity of the sensor.
According to another characteristic of the invention, the pressure inside the cladding is measured continuously for two equal durations shorter than the time taken by the rising edge to go past, and separated by a time interval shorter than the time taken by the rising edge to go past, the curve of the measurements taken during each of said durations is integrated, the difference between the two integration values is determined, and the pressure difference obtained is compared with a threshold value.
The pressure of the dielectric gas is monitored continuously, and this algorithm makes it possible to measure two values of the pressure at two close-together instants, and the resulting value is independent of the steady pressure value which can itself be subject to fluctuations. As a result, accuracy is significantly improved, and therefore so is the sensitivity with which the passing pressure surge is detected.
Advantageously, the measurement durations are substantially equal to one half of the time taken by the rising edge to go past, and the time interval is substantially equal to one half of the time taken for the rising edge to go past. These times depend on the propagation speed of pressure waves in the dielectric gas.
This makes it possible to compare pressure values during two durations corresponding respectively substantially to the beginning of the rising edge and to the end thereof so that the difference between the integrated values in maximized.
According to another characteristic of the invention, the detection is performed substantially in the middle of the cladding segment to be monitored.
In this way, sensitivity is further improved because the attenuation of the pressure surge as it propagates is limited to the attenuation corresponding to one halt of the length of the segment to be monitored.
The invention also provides a device for implementing the above-mentioned method, the device comprising at least one one-way detection unit constituted by a pressure sensor disposed outside the cladding and connected to the inside the cladding via a horn-shaped duct which slopes relative to the axis of the cladding, which has its mouth opening out in the cladding, and which has its tip connected to the pressure sensor.
The resulting interface between the segment to be monitored and the sensor makes it possible firstly to improve sensitivity because of the additional surge pressure generated by the duct which constitutes a sort of funnel. Furthermore, the resulting detection unit acts in one-way manner and is sensitive only to the pressure surges coming from the direction opposite to the direction in which the duct slopes.
Advantageously, there is provided a set of at least two one-way detection units having the same detection direction and disposed in the vicinity of each other, and whose ducts slope the same way.
This configuration makes it possible to obtain redundancy in terms of fault location by mitigating sensor failure.
It is possible to provide two one-way detection sets whose ducts slope in mutually opposite directions and which are disposed in the vicinity of each other.
In this way, by using four detection units, it is possible to detect and to locate, with good sensitivity and high reliability, faults that occur on either side of the detection sets.
In an embodiment of the invention, use is made of two pressure sensors connected in parallel to two ducts which slope in mutually opposite directions and whose tips communicate with each other.
This configuration makes it possible to form a both-way detector device of high sensitivity and having detection redundancy because the sensors are duplicated.
In another embodiment of the invention, use is made of two directional detection units whose ducts slope in mutually opposite directions
A both-way detection device is thus obtained that makes it possible to locate the faults but that has no back up against sensor failure.
Advantageously, for these two embodiments, use is made of the two sensors serving to monitor the density of the dielectric gas. This makes it possible to form a detection device at little extra cost.